Altering keyframe frame rate...

I have a character animation going on in AE. The character is made up of lots of layers, some having different blending modes which react in different ways to various glowing objects beneath the character.

What I want to do is create a slightly jerky, stop-motion look to the character alone, leaving everything else normal.

Usually I'd just precomp the entire character and apply posterize time. However, taking that approach in this case messes up all the blending modes. Also, there are particular layers, shine gobos etc that rely on some of the character layers being 'not-precomped'.

So assuming precomping is not an option - is there a way to get this effect (posterizing time to, say, 12 fps) via an expression used on the various keyframes? Or another approach I can try? The effect would have to be applied to multiple layers...